This invention relates to a circuit arrangement for separating high-frequency pulses generated successively in time by a piezoelectric transducer of an acoustic reflecting lens arrangement.
Acoustical microscopy can be carried out within a frequency range of from 50 to 5,000 MHz. In acoustical reflection microscopy, short pulses of this frequency having a duration of about 10-100 nsec and a pulse repetition frequency of about 500 kHz are used.
The most important component of the acoustical microscope is the acoustic lens arrangement. This generally comprises a peizoelectric transducer which converts electric energy into acoustic energy and back again, and a lens surface in the form of a spherical cavity in the surface of the acoustic transmission medium opposite to the transducer. The acoustic transmission medium is selected to have a high sound velocity in order to reduce spherical aberrations when the acoustic lens is coupled to a liquid transition medium. Sapphire is normally used as transmission medium and distilled water as coupling means.
The piezoelectric transducer can be optimized for a predetermined frequency range, but its bandwidth will not exceed one or two octaves. This means that the acoustic lens arrangement must be exchanged if it is intended to change the required operating frequency. It is desirable, however, that the electronic circuit driving the acoustic lens arrangement in the form of short pulses and receiving the echo pulses from the object can be kept unchanged. This is because the amount spent on the circuitry for an acoustic lens arrangement is a significant factor in the overall system cost.
The necessary electronic circuitry is also complicated because the internal reflections in the acoustic transmission medium are rather large and the echo pulses from the object, which are the actual item of interest, are relatively small due to the high attenuation in the liquid coupling medium. For this reason, it is necessary to insert between the output of the circuit generating the excitation pulses, the input of the circuit used for processing the measurement signal, and the transducer of the acoustic lens arrangement, an interface which separates the high-frequency pulses occurring successively in time so that all three electronic components can be considered as being largely decoupled from each other.
A simple circuit element which, however, only incompletely solves this problem, is a microwave circulator which, in a first phase, guides the excitation pulse to the transducer, then blocks the entrance and opens the conduction path from the transducer to the measurement signal processing circuit. It must be noted that such a circulator does not allow complete line isolation. Thus, a not negligible portion of the excitation pulse always passes directly to the measurement signal line. On this leakage signal, electric reflections are superimposed which are produced by a mismatch between the characteristic impedances of the feeder to the transducer and of the transducer itself. This signal, which is generated first in time, shall be designated by A.
Despite anti-reflection coating of the boundary area, a second signal B is produced by internal reflection of the excitation pulse at the boundary area between the acoustic transmission medium and the liquid coupling medium. This second signal is typically 10-20 dB smaller than pulse A and is delayed with respect to this pulse by a time t.sub.1 which is determined by the velocity of propagation in the acoustic medium and the length of the transmission path.
A signal C representing the pulse reflected at the object follows with a further time delay t.sub.2. Its magnitude is considerably smaller than all other signals because of attenuation in the liquid coupling medium. Its amplitude is determined by the shape of the lens since this determines the transmission path in the liquid, by the operating frequency, and by the attenuation parameters of the liquid. The amplitude of signal C is typically 30-90 dB lower than that of signal A.
Internal reflection inside the lens body can also produce a signal D which has a time delay t.sub.1 with respect to signal B.
Since the circulator is incapable of separating the signals A, B, C, and D, difficulties arise in isolating the signal C, the only signal which is actually of interest. Also, the amplifiers available in practice have a long post-saturation recovery time. As a rule, the linearity of the amplifiers has not yet been restored when signal C arrives since signal B is considerably larger than signal C and the latter follows the former very rapidly.
Another very important problem arises as a result of the abovementioned mismatch of the impedances on the feeder to the transducer. Every signal entering this conduction path tends to be reflected back and forth for a long time. For constructional reasons, the length of this feeder cannot be made arbitrarily small. This is why the duration of the electric reflections on this line can be as long as a time interval t.sub.2 sufficient to cause undesirable problems of interference with the object signal C.
From Electronics Let., Vol. 14 (1978), pp. 472-473, a circuit arrangement is known which is intended to be used for suppressing the leakage signal from the circulator and interfering echo signals from the acoustic lens arrangement. For this purpose, a switch is inserted into the measurement signal input behind the circulator, which switch is opened only after a time corresponding to the time of transit of the excitation pulse to the specimen and back to the switch. Pin switches are suitable switches for such purposes. For the proposed system, a very clean and thus expensive switch with a minimum of switching spikes must be selected since otherwise a following amplifier would be overdriven by the switching spikes, which contain very high frequency components. The system has the further disadvantage that it is virtually impossible to eliminate the previously-mentioned reflections on the feeder to the transducer. In addition, the possible frequency range for the excitation pulses and thus the working frequencies for the acousto-microscopic examination, which range can be covered by exchanging the acoustic lens system, is limited by the characteristics of the circulator.
From J. Appl. Phys. Vol. 50 (1979), pp. 1245-1249, a circuit arrangement is known in which the circulator is replaced by a hybrid coupler. Such a coupler is a passive component in which electric signals can propagate only in predetermined directions. The attenuation in the directional conductors is relatively large. The signal losses are about 6 dB. Capacitive coupling between individual directional conductors frequently results in signal crosstalk.
The known hybrid coupler is arranged in such a way that a direct passage exists between the high-frequency generator and the transducer of the acoustic lens arrangement, and a further directional conductor from the transducer to the measurement signal processing circuit. A switch controlled with time delay is again inserted into this line so that the interfering echo signals can be suppressed. From the hybrid coupler input another directional conductor extends which is terminated by a 50-Ohm resistor. This makes it possible to attenuate reflections produced by mismatch on the line from the high-frequency generator to the hybrid coupler. Here, too, the problem of the reflections on the line from the transducer to the measurement signal input switch and of suppression of the switching spikes of this switch has not been solved.
Another circuit arrangement for suppressing the undesirable echo signals is specified in European Pat. No. 0.032,732. The solution proposed in that patent consists in arranging a second acoustic lens next to the acoustic lens arrangement used for object scanning, the second acoustic lens having the same constructional data and a reference object. Subtraction of the two signal sequences returning from the lenses is intended to leave only the cleaned-up measurement signal. Because of the second lens arrangement, this arrangement is very expensive and presents great difficulties in implementing identical lens parameters.